Among NOx purification catalysts (DeNOx catalysts) used for purification of NOx in exhaust gas from diesel engines, in-cylinder gasoline direct injection engines (GDI), and the like, there is a NOx occlusion-reduction catalyst called a lean NOx trap (LNT). This catalyst is formed by supporting an occlusion material such as an alkali metal (for example, potassium K, or the like) or an alkaline earth metal (for example, barium Ba, or the like) together with a noble metal such as platinum Pt. In an air-fuel ratio lean state where the exhaust gas is oxygen-rich, the catalyst oxidizes NO in exhaust gas, and occludes NOx in the form of a nitrate salt of the occlusion material. Meanwhile, in an air-fuel ratio rich state where the exhaust gas contains almost no oxygen, the catalyst releases the occluded NOx, and reduces the released NOx with a reducing agent such as HC or CO by way of a three-way catalyst function. By way of these functions, the catalyst reduces the amount of NOx in exhaust gas.
To restore the NOx occlusion capacity of the NOx occlusion-reduction catalyst, NOx regeneration control is conducted. In the NOx regeneration control, when the air-fuel ratio of exhaust gas is brought into a rich state, a reducing agent such as hydrocarbon (“HC”) is supplied to the NOx occlusion-reduction catalyst. The supply of the reducing agent is achieved by performing post-injection in which fuel is injected into a cylinder after main injection, or by performing in-exhaust pipe fuel direct injection in which fuel is injected directly into an exhaust pipe.
However, the supply of HC involves a problem that the HC may pollute the atmosphere. Specifically, when the HC is supplied too much to be consumed by the NOx occlusion-reduction catalyst, or when the temperature of the NOx occlusion-reduction catalyst is low, HC not used in the reduction of NOx outflows to the downstream side, and HC slip, which is the outflow of HC to the atmosphere, occurs.
As a countermeasure against this problem, the amount of the noble metal supported is increased, or ceria (cerium oxide) which functions as an oxygen occlusion material is used as an auxiliary catalyst, for strengthening the HC oxidation function of the NOx occlusion-reduction catalyst. However, the countermeasure is unsatisfactory as a countermeasure against the outflow of HC.
Meanwhile, an exhaust gas purification apparatus for an internal combustion engine configured as follows to prevent the emission of hydrocarbons to the atmosphere has been proposed as described in, for example, Japanese patent application Kokai publication No. 2000-257417. The exhaust gas purification apparatus is constituted of a NOx occlusion-reduction catalyst, an HC-adsorbing member capable of preferentially adsorbing heavy hydrocarbon in exhaust gas, and an oxidation catalyst which are disposed on an exhaust passage in this order from the upstream side. Here, when a reducing agent is supplied at low temperature, the HC-adsorbing member adsorbs heavy components of hydrocarbon which have not reacted on the NOx occlusion-reduction catalyst, and soft components of hydrocarbon are purified with the oxidation catalyst. In addition, after the temperature rises, hydrocarbon released from the HC-adsorbing member is purified with the oxidation catalyst.
However, the amount of HC adsorbed may be not so much in some cases even when the temperature of the HC-adsorbing member is low, and HC cannot necessarily be adsorbed sufficiently by the HC-adsorbing member. Accordingly, there has been a problem that the outflow of HC to the downstream side of the exhaust gas purification apparatus cannot be prevented sufficiently. Moreover, the inventors of the present invention have found through a lot of experiments that the HC adsorbing capacity of the HC-adsorbing member has a relationship not simply with the temperature of the HC adsorbing agent, but also with the oxygen concentration in the gas containing HC.